The present invention relates to a method and a device for determining a characteristic value of a wheel brake.
In modern control systems for brake systems, brake-specific characteristic values are often used. One essential characteristic value is the proportionality constant between the braking torque exerted on the wheel brake, i.e., the braking force applied, and the braking pressure established. This variable is used, for example, with the known vehicle dynamics control systems. Such a control system is described in the publication, SAE paper 950759 “VDC—the Vehicle Dynamics Control System of Bosch” by Anton van Zanten, Reiner Erhardt and Georg Pfaff. The setpoint braking torques for a wheel determined by the control system described there are converted into setpoint pressure values by using a proportionality factor CP, where proportionality factor CP describes the brake-specific correlation between the braking torque and the brake pressure. Furthermore, the brake characteristic values are also used in the hydraulic model to determine the brake pressures from the control signals for the valves, so that the characteristic values have an effect on the accuracy of the control. The characteristic values play a role in determination of the wheel performance (e.g., in methods of calculating slip, etc.) and variables derived therefrom.
The same thing also applies to electrically controlled brake systems in which the braking force applied to the wheel, i.e., the braking torque, is adjusted by an electronic regulator as a function of the degree of operation of an operating element, in particular a brake pedal, operable by the driver. One example of this is German Patent Application 196 48 936 A1, where the brake characteristic values are also analyzed in controlling the wheel brakes.
It has been found that the brake characteristic values of wheel brakes are not constant but instead vary as a function of temperature, the age of the brakes, and the condition of the brake disk. The reason for the change is that these circumstances result in a change in the coefficient of friction between the brake shoe and the brake disk or drum and thus influence the relationship between the brake pressure and the braking force applied, i.e., the braking torque. If brake characteristic values were assumed to be constant, this would result in inaccuracies, which are unwanted in some applications, in the calculations using these characteristic values or it would result in an incorrect appraisal of wheel performance.
Therefore, German Patent Application 196 48 936 A1 cited above describes a procedure with the help of which the brake characteristic value and/or values are adapted to the changes. On the basis of the equilibrium of forces of the vehicle in the longitudinal direction, a correlation is derived there according to which the prevailing brake characteristic value is determined on the basis of the brake pressure and the deceleration of the vehicle. With the method proposed there, the brake characteristic values may be determined only for an axle.